A very interesting talk by Rich Hikey (creator of Clojure) on the topic: https://m.youtube.com/watch?v=oyLBGkS5ICk

To summarize, any change that offers more (as in, more fields in the output) and/or requires less (more of the original parameters made optional) is non-breaking.

Edit: Of course, this is all under the assumption that the semantics of methods should never change. Instead of changing semantics, he proposes to just create a new method with a new name.